The present invention relates to resistive thin films, particularly to metal-oxide thin film resistors, and more particularly to Ti--Cr--Al--O thin film resistors and a process for fabricating same.
The development of metal-oxide materials has been widely pursued in the electronics industry for use as resistive thin films. The use of multiple phases provides a path to change the film resistivity. See C. A. Neugebauer, "Resistivity of Cermet Films Containing Oxides Of Silicon", Thin Solid Films, 6 (1970), 443-447. The dependence of sheet resistivity on composition is well established for systems such as Cr--Si--O. See. R. Glang et al., "Resistivity and Structure of Cr--SiO Cermet Films", J. Vac. Sci. Technol., 4 (1967), 163-170; A. A. Milgram et al., "Electrical and Structural Properties of Mixed Chromium and Silicon Monoxide Films", J. Appl. Phys., 39 (1968), 4219-4224; N. C. Miller et al., "Co-sputtered Cermet Films", Solid State Tech., 11 (1968), 28-30; and H. Steemers et al., "Stable Thin Film Resistors For Amorphous Silicon Integrated Circuits", Mat. Res. Soc. Symp. Proc., 118 (1988), 445-449. The conduction mechanism for these cermet materials (materials composed of ceramics and metals) can be considered quantum mechanical. See. J. E. Morris, "Structure and Electrical Properties of Au--SiO Thin Film Cermets", Thin Solid Films, 11 (1972), 299-311. For low metallic concentrations, the charge transport is proposed to be by electron tunneling between the metallic particles. See B. E. Springett, "Conductivity Of A System Of Metallic Particles Dispersed In An Insulating Medium", J. Appl. Phys., 44 (1973), 2925-2926. In general, conduction may be considered to be by means of an activated charge transport process. For film resistivities &gt;10.sup.-2 Ohm-cm, the microstructure is usually comprised of a continuous insulating matrix in which metallic particles are dispersed. An increase in metallic content produces a decrease ion sheet resistivity. For the Cr--Si--O system, the insulating matrix is based on the oxide phase of SiO.sub.2, with Cr, silicides, and monoxides serving as conductors/semiconductors. A general observation by Neugebauer, Supra, suggests that the SiO.sub.2 composition alone could be used to determine the cermet film resistivity to within two orders of magnitude irrespective of deposition technique or conditions. Whereas this summation may represent a general trend, it is not an inclusive statement for the resistivity behavior of Cr--Si--O cermets. Initial work at the Lawrence Livermore National Laboratory with the Cr--Si--O cermet system has shown a widely varying range of resistivities that span more than twelve-orders of magnitude and are often accompanied by a non-linear current-voltage behavior. See A. Jankowski et al., "Resistivity Behavior Of Cr--Si--O Thin Films", Chem. Phys. Nanostructures and Related Non-Equilibrium Materials, ed. E. Ma. et al., The Minerals, Metals and Materials Soc. Proc. (1997), pg. 211-219. In addition, post-deposition vacuum annealing can cause changes in the resistivity by several orders of magnitude rendering unreliable use of the Cr--Si--O film as a resistor layer of constant value. Due to the limitations of producing a consistent resistivity from 10.sup.5 to 10.sup.8 Ohm-cm for the Cr--Si--O system, an alternate material has been sought which would have a well-defined and stable behavior as a resistor layer.
The present invention provides the sought for alternate for the Cr--Si--O system, and it has been determined that the system of the present invention has a well-defined and stable behavior as a resistor layer. The Ti--Cr--Al--O cermet of the present invention is being developed for use as a thin film resistor since its properties in bulk form are favorable and controllable. The Ti--Cr--Al--O films are radio frequency (rf) sputter deposited to transfer the target composition to the growing cermet film. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O.sub.2. The film resistivity can be discretely selected through target composition and the control of the deposition parameters.